Prosthetic wrist implant

ABSTRACT

A wrist implant requires minimal resection of the distal radius and preserves the sigmoid notch and articulation with the head of the distal ulna. The wrist implant generally includes a radius portion, a carpal portion and a carpal ball. The wrist implant includes a primary articulation and a secondary rotational articulation. The primary articulation occurs between the radius portion and the carpal ball. The secondary articulation occurs between the carpal ball and the carpal portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application claiming priority fromapplication Ser. No. 10/897,317 filed Jul. 22, 2004, which claimspriority from U.S. Provisional Patent Application Ser. No. 60/489,037filed Jul. 22, 2003.

FIELD OF THE INVENTION

The invention relates to prosthetic orthopedic implants. Moreparticularly the invention relates to a prosthetic orthopedic wristimplant for prosthetic replacement of a damaged, diseased or degeneratednatural wrist joint.

BACKGROUND

Orthopedic replacement of damaged or degenerated natural wrist joints iswell known in the orthopedic arts. Prior to the introduction ofprosthetic joint replacement for the wrist, individuals suffering from ajoint disease in the wrist such as radio-carpal arthritis were oftensurgically treated by a fusion procedure. Fusion involves repairing theinjured wrist joint structures with a fixed plate or rod that stiffensthe wrist. That is, the joint is fixed in position by a device thatallows no movement of the wrist. While this was an improvement over adiseased or injured wrist joint it is clearly unsatisfactory.

Existing orthopedic prostheses for wrist joint implantation have anumber of limitations. Currently, most prosthetic wrist implants providethe patient with only limited functionality of the wrist, otherwise theimplant becomes unstable. The natural wrist is astonishingly flexible inits freedom of motion. If a prosthetic wrist implant does not providesufficient motion in flexion, extension, radial deviation, or ulnardeviation, the patient may have difficulty performing many of the normaltasks of daily living. Ideally, after implantation of a wristprosthesis, the wrist will have a range of motion equal to or at leastapproaching that of a natural wrist joint.

An important requirement for prosthetic wrist implants is to have anextremely secure attachment between the implant and the bones.Separation of the prosthetic wrist implant from the bones to which ithas been secured can be a serious complication requiring a repeatsurgical procedure to make repairs. Failure of the attachment betweenthe wrist prosthesis and the bones to which it is attached will causefurther damage to the bones in some circumstances, making it difficultor impossible to treat the wrist even with a replacement implant.Prosthetic wrist implants currently in use generally require resectionof the peripheral rim of the distal radius with its important ligamentsand soft tissue attachments. The loss of these ligamentous attachmentstends to create instability (looseness) of the prosthetic wrist. Tocompensate for this instability, a more involved surgical procedure mustbe performed to make reattachments of the soft tissue to the bone.

In addition, some currently available prosthetic wrist implants requirethe resection of a substantial amount of bone from the carpal bonestructures. This substantial resection relocates the normal wristcenters of rotation (or motion) and relocation of the centers of wristrotation interferes with normal function of the wrist extensor andflexor tendons, alters tendon moment arms and, as a result, limits andweakens movement of the wrist in extension and flexion. Further, if apatient later needs another procedure at the same joint revision optionsare limited if excessive tissue has been resected.

Another shortcoming of existing prosthetic wrist implants is thelimitation of torsional movement of the wrist related to the elbow. Thehealthy hand and wrist are able to rotate about an axis generallyparallel to that of the long axis of the forearm, both because of therotation of the radius and the ulna about one another, and because ofthe rotation of the natural wrist bones with relation to the radius andthe ulna. Currently available prosthetic wrist implants typicallyinvolve the secure attachment of a proximal component of the wristimplant to the distal end of the radius and a distal component to thecarpus with fixed planes of motion that result in a loss of torsionalrange of motion. In addition, if the forces involved in torsionalmovement of the wrist are limited by the implant as with currentdesigns, those forces are transferred to the bone-implant interfaces atthe radius and the carpus, increasing the risk of the implant loosening,and contributing to implant failure.

Further, in some wrist implant designs, a single stem extends throughthe carpus and into one or more of the metacarpals. The distal componentof these wrist implants tends to erode through the metacarpal bone andcreate instability of the carpal attachment. Consequently the distalcomponent of the implant may loosen or fracture where the implant entersthe bones of the hand. In addition, in the normal wrist there is somefreedom of motion between the carpals and the metacarpals and a stempassing through the carpals and into the metacarpals limits that freedomof motion resulting in less than ideal function of the wrist afterimplantation.

In addition, it has been found that implants that allow metal to metalcontact between the radial and carpal components tend to cause sheddingof metal particles that may migrate into surrounding tissues and maycause tissue necrosis and consequent implant failure and othercomplications.

Thus, it would be valuable to provide an improved orthopedic wristimplant that would provide a range of motion simulating the naturalwrist's range of motion as closely as possible. In addition, it would bedesirable if a prosthetic wrist implant would provide an improvedtorsional range of motion and reduce the effect of torsional forces onthe bone-implant interface. It would further be desirable that aprosthetic wrist implant provide a secure, strong, and stable attachmentto the surrounding bones in order to provide a wrist implant that wouldhave low complications related to implant loosening. Further, it wouldbe beneficial to preserve the peripheral rim of distal radius as well asthe sigmoid notch of the distal radius where it articulates with thehead of the ulna. It would be preferable to avoid metal-to-metal contactbetween the radial and carpal components.

SUMMARY OF THE INVENTION

The prosthetic wrist implant of the present invention solves many of theabove limitations and problems related to wrist implant failure. Thewrist implant of the present invention requires little or no resectionof the distal radius and minimal resection of the carpal bones of thewrist. The implant is available in left and right hand configurationswith geometrically scaled sizes that approximate the anthropomorphicsizes of different radio-carpal joints. The wrist implant generallyincludes a radius portion, a carpal portion and a carpal ball. Theradial component of the prosthetic wrist implant is designed much like asurface replacement arthroplasty (SRA) in that it seats against thescaphoid and lunate fossae and preserves the peripheral rim of thedistal radius with its important ligamentous and soft tissueattachments. This configuration requires minimal or no resection of thedistal radius and preserves the sigmoid notch and articulation of thedistal radius with the head of the distal ulna.

The carpal portion of the wrist implant is a low-profile design thatminimizes the amount of bone resection and does not interfere with thenormal function of the wrist extensor and flexor tendons. The carpalportion of the wrist implant may include a central stem for insertioninto the capitate bone. The carpal portion accommodates two carpalscrews for fixation to the scaphoid and hamate bones within the distalcarpal row.

A carpal ball component preferably acts as an intercalated segment thatarticulates with both the radius and carpal portions of the implant. Theprimary articulation occurs with the radial component. The primaryarticulating surfaces may be ellipsoidal and toroidal and act along twoperpendicular axes of rotation. The first axis lies in the coronal planeand the second axis lies in the sagittal plane thus permitting motion inflexion-extension and radial-ulnar deviation. The concavity of theradial portions' articular geometry resists ulno-volarly directed forcesthat can cause excessive wear and implant subluxation or dislocation.

In one embodiment a secondary articulation of the carpal ball componentoccurs with a carpal plate. This articulation is rotational and occursabout an axis aligned generally parallel with the longitudinal axis ofthe third metacarpal bone. This additional degree of freedom divertstorsional forces from the bone implant interfaces, thus reducing therisk of implant loosening and lessening the risk of implant failure.This additional degree of freedom of movement also compensates forpotential misalignment of the implant due to advanced deformity of theinjured wrist caused, for example, by rheumatoid or degenerativearthritis.

The interface between the ellipsoidal carpal ball and the toroidalradial component allows flexion-extension and radio-ulnar motion tooccur about different axes of rotation. This allows for a hinge likemotion dorso-palmarly and a gliding motion radio-ulnarly closelyapproximating natural joint kinematics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prosthetic wrist implant in accordancewith the present invention;

FIG. 2 is a perspective view of the prosthetic wrist implant from areverse angle to that of FIG. 1;

FIG. 3 is an exploded view of the prosthetic wrist implant;

FIG. 4 is a exploded perspective view of the prosthetic wrist implantfrom a reverse angle to that of FIG. 3;

FIG. 5 is a plan view of a radius component in accordance with thepresent invention;

FIG. 6 is an elevational view of the radius component in accordance withthe present invention;

FIG. 7 is a perspective view of the radius component;

FIG. 8 is a second perspective view of the radius component;

FIG. 9 is a plan view of a carpal screw in accordance with the presentinvention;

FIGS. 10 a and 10 b are perspective views of two carpal screws inaccordance with the present invention;

FIG. 11 is a plan view of a carpal ball in accordance with the presentinvention with phantom lines depicting internal structure;

FIG. 12 is an elevational view of the carpal ball with phantom linesdepicting internal structure;

FIG. 13 is a perspective view of the carpal ball;

FIG. 14 is a second perspective view of the carpal ball in accordancewith the present invention;

FIG. 15 is a plan view of a carpal plate in accordance with the presentinvention;

FIG. 16 is an elevational view of the carpal plate in accordance withthe present invention;

FIG. 17 is an end view of the carpal plate in accordance with thepresent invention;

FIG. 18 is a perspective view of the carpal plate in accordance with thepresent invention;

FIG. 19 is a perspective view from a second angle of the carpal plate inaccordance with the present invention;

FIGS. 20 a and 20 b are exploded plan views showing the right and lefthand configurations of the prosthetic wrist implant;

FIG. 21 is a plan view showing the articulation of one embodiment of acarpal plate and carpal screw in accordance with the present invention;

FIG. 22 is a plan view of the articulated parts of a prosthetic wristimplant;

FIG. 23 is an elevational view of the articulated parts of theprosthetic wrist implant;

FIG. 24 is an end view of the articulated parts of the prosthetic wristimplant;

FIGS. 25 a and 25 b depict the tertiary articulation of the carpal plateand carpal ball in accordance with one embodiment of the presentinvention;

FIGS. 26 a-d depict the radial deviation, extension, ulnar deviation,and flexion of the primary articulation of an embodiment of the presentinvention; and

FIG. 27 is a schematic of the prosthetic wrist implant as implanted in ahuman wrist.

DETAILED DESCRIPTION OF THE DRAWINGS

A wrist implant 30 of the present invention generally includes radiuscomponent 32, carpal ball 34, carpal plate 36 and one or more carpalscrews 38. Referring to FIGS. 1-4 radius component 32 articulates withcarpal ball 34 via a primary articulation 40. Carpal ball 34 articulateswith carpal plate 36 via secondary articulation 42 as best seen in FIG.25. Carpal screws 38 articulate with carpal plate 36 via tertiaryarticulation 44, as best seen in FIG. 21.

Referring, in particular, to FIGS. 5-8, radius component 32 generallyincludes intra-medullary stem 46 and articular cup 48. In oneembodiment, intra-medullary stem 46 is generally quadrilateral incross-section and tapers from broad at a first end 50 to narrower atsecond end 52. Preferably, intra-medullary stem 46 and articular cup 48are formed, cast or machined as an integral unit from a single piece ofmaterial. The juncture 54 between intra-medullary stem 46 and articularcup 48 is tapered in a fillet 56. Each corner 58 of intra-medullary stem46 is radiused, beveled or chamfered. Intra-medullary stem 46 furtherdefines rounded end 60 and a straight portion 62. Intra-medullary stem46 also defines a stem thickness D. Intra-medullary stem 46 has a smoothcontinuous surface throughout and may be surface treated to encourageosseointegration. Thus, stem geometry emulates the intra-medullarycontour of the distal radius. The smooth continuous surface ofintra-medullary stem 46 means that medullary stem 46 lacks sharp cornersor any significant discontinuities. The smooth continuous surface ofintra-medullary stem 46 may include roughening of texture to encourageosseointegration while still retaining a smooth continuous shape.

Articular cup 48 is generally toroidal in shape and has a majordimension A and a minor dimension C. Articular cup 48 defines agenerally toroidal concave articular surface 64. Articular surface 64preferably has two perpendicular axes, one axis of rotation and oneinstantaneous center, which lie in generally coronal and sagittalplanes. Articular surface 64 defines two radii, a major radius, R1 and aminor radius R2. Major radius R1 and minor radius R2 may be selected toemulate the curve traced by the proximal portion of the scaphoid-lunatecomplex in the normal articulation of the human wrist as is clearly seenin FIG. 26. Articular cup 48 further includes cup wall 66. Cup wall 66is preferably of generally uniform thickness. Cup wall 66 is desirablycurved to conform to the curvature of the scaphoid and lunate fossae ofthe distal end of the radius. Thus, the curvatures of articular surface64 have generally shorter radii than does the exterior of cup wall 66.Articular cup 48 further defines dorsal and volar cutouts 67. Thesurface at dorsal and volar cutouts 67 is highly polished to facilitatethe excursion of the flexor and extensor tendons and other soft tissuesthereover. Dorsal and volar cutouts 67 also maximize range of motion inflexion and extension without metal to metal impingement of the radialand carpal components.

Articular surface 64 is, desirably, precision machined and highlypolished for articulating with carpal ball 34. Radius component 32 ispreferably fabricated from cobalt chrome-molybdenum alloy material. Thejuncture between intra-medullary stem 46 and articular cup 48 is angledto approximate the anatomical volar tilt and ulnar inclination angles ofthe distal radius. The articular surface 64 of the articular cup 48 isangled volarly and ulnarly. Ulnar inclination may be about twenty totwenty two degrees and volar tilt may be about ten to twelve degrees.

Intra-medullary stem 46 is offset in the anterior-posterior and lateralplanes to align the articular cup 48 to seat against the lunate andscaphoid fossae and preserve the distal ulna. The intra-medullary stem46 may be coated with a commercially pure titanium plasma coating topromote osseointegration.

Referring in particular to FIGS. 11, 12, 13 and 14, carpal ball 34 isgenerally ellipsoidal in shape. Carpal ball 34 has a convex articularsurface 68. Convex articular surface 68 may be shaped to generally matcharticular surface 64 of articular cup 48 to provide a close sliding fittherewith. Carpal ball 34 defines slightly flattened but still roundedarticular ends 70. Convex articular surface 68 has a major radius R1 anda minor radius R2 generally matching, respectively, those of articularcup 48. Carpal ball 34 also has a back surface 72. Back surface 72 iscurved along a relatively flat secondary spherical radius SR1. Backsurface 72 further defines socket 74, and, preferably, two ovalexcavations 76. Socket 74 includes rim 78 and spheroidal portion 80.Socket 74 is, desirably, generally spherical in shape having a rim 78that is of a lesser diameter than spheroidal portion 80.

Carpal ball 34 has a major dimension B and a minor dimension D. Carpalball 34 also presents a major curvature and a minor curvature.

Preferably, there are two oval excavations 76 on back surface 72. Ovalexcavations 76 are of an oval, ellipsoidal, arcuate or racetrack shapeand of a generally uniform depth. Oval excavations 76 may be arcuatecentered about socket 74.

Carpal ball 34 is made from ultra high molecular weight polyethylene(UHMWPE) or another self lubricating material. UHMWPE has the advantageof providing a thermal break as well as minimizing wear on the otherarticular components. However metals or other materials may also beutilized. Carpal ball 34 may be machined, molded or formed by othertechniques known to the art.

In another embodiment of the invention, articular cup 48 defines agenerally toroidal concave articular surface 64 that allows slight playbetween articular surface 64 and convex articular surface 68. In thisembodiment, carpal ball 34 has a convex articular surface 68 that tapersslightly toward articular ends 70. Convex articular surface 68 is shapedto match articular surface 64 of articular cup 48 to provide a closesliding fit therewith near the center of convex articular surface 68 buta looser sliding fit near articular ends 70. It is important to notethat the ellipsoidal geometry of the carpal ball 34 demonstrates aline-to-line contact of the carpal ball 34 and the articular Clip 48 inthe anterior-posterior and lateral planes.

Referring in particular to FIGS. 15-19, carpal plate 36 generallyincludes plate 82, carpal stem 84 and ball 86.

Plate 82 includes buttress surface 88, secondary articular surface 90and defines screw holes 92. Buttress surface 88 is preferablysubstantially flat. Secondary articular surface 90 is curved alongspherical radius SR1 to match back surface 72 of carpal ball 34. Thisfacilitates better wear characteristics between carpal ball 34 andcarpal plate 36 by reducing peripheral stress risers that may causefretting of the carpal ball 34 and, possibly, debris generation. Screwholes 92 pierce plate 82, preferably on opposite sides of ball 86. Screwholes 92 include spheroidal countersink 94 and cylindrical rim 95. Plate82 is generally elliptical in shape as best seen in FIG. 17. Theperiphery of plate 82 is radiused at the edge of secondary articularsurface 90. Distal to the radiused edge the periphery of plate 82 isdrafted to optimize articular contact area and avoid metal to metalcontact at the extremes of range of motion and axial rotation.

Carpal stem 84 extends outwardly from bone interface surface 88 adistance E and has a diameter D2 at its proximal end, having a smooth orstepped taper to D1 at its distal end. Preferably, carpal stem 84 isgenerally perpendicular to plate 82. Carpal stem 84 may includeretaining ridges 96 and hemispherical end 98. Referring particularly toFIGS. 16-17 and 19, note that carpal stem 84 may be offset from a lineconnecting screw holes 92 a distance G. Carpal stem 84 is of a length Esuch that it does not extend into or beyond the carpal-metacarpalinterface.

Ball 86 is secured against secondary articular surface 90. Preferably,plate 82, carpal stem 84 and ball 86 are integrally formed or machinedfrom a single piece of material. Ball 86 is spheroidal in shape and hasa diameter larger than that of ball support 100. Ball 86 is locatedgenerally between screw holes 92 to fall, when implanted, along an axisgenerally aligned with the third metacarpal. The diameter of ball 86 isequal to or slightly smaller than spheroidal portion 80 of socket 74 ofcarpal ball 34.

Carpal plate 36 is advantageously fabricated from cobaltchrome-molybdenum alloy material. Buttress surface 88 and carpal stem 84may be coated with commercially pure titanium plasma coating to promoteosseointegration. As mentioned above, carpal stem 84 is dorsally offsetfrom screw holes 92 to accommodate the arch of the carpus. Secondaryarticular surface 90 and ball 86 along with ball support 100 areprecision machined and highly polished to allow smooth articulation withcarpal ball 34.

Referring particularly to FIGS. 9-10 and 21, carpal screws 38 generallyinclude head 102 and shaft 104. In one embodiment, head 102 isspheroidal in shape and defines flat face 106 and driver interface 108.Driver interface 108, as depicted, accepts a standard 2.5 mm hexscrewdriver but can be formed to interface with any screwdriver known inthe art. Carpal screws 38 are selected of a length so that carpal screws38 do not cross the carpal-metacarpal joint when implanted.

Shaft 104 may be of a length and diameter desired within the abovelimitations and has a cancellous thread form 110 for optimum fixationwith the carpal bones of the wrist. An unthreaded portion of shaft 104provides clearance so that carpal screws 38 may conically rotaterelative to carpal plate 36. Spheroidal portion 112 of head 102 allowsfor ideal screw angulation as determined by a surgeon during thesurgical procedure. Preferably carpal screws 38 are placed into thescaphoid and hamate bones of the carpus. The radius of spheroidalportion 112 is equivalent to that of spheroidal countersink 94 of carpalplate 36. Spheroidal portion 112 stands proud of secondary articularsurface 90 of carpal plate 36 when implanted.

Carpal screws 38 are fabricated from cobalt chrome-molybdenum alloymaterial. Cancellous thread form 110 is designed to provide a securegrip in the cancellous and cortical portions of the carpal bones. Carpalscrews 38 may be fluted for self-tapping application. The length ofcarpal screws 38 may vary as needed to accommodate patient anatomy butadvantageously should not pass into or through the carpal-metacarpaljoint. Carpal screws 38 may be manufactured to the standards of ISO5835.

In operation, wrist implant 30 is implanted in the wrist of a patient toreplace damaged or degenerated wrist structure as depicted in FIG. 27.In general, radius component 32 is implanted in the distal end of theradius without the need to perform resection of the distal portion ofthe radius. The distal radius cartilage and any heterotopic bone orosteophytes are removed from the scaphoid and lunate fossae at the endof the distal radius, but the bony structures of the distal radiuspreferably are not resected at all. A starter hole is drilled in thedistal radius generally parallel to the long axis of the radius. Thisdrilled hole provides a starting point for the use of a series ofprogressively larger broaches until the cavity in the distal radius islarge enough to allow full seating of the radius component 32. Thecavity is broached until it is large enough to provide a press fit forintra-medullary stem 46.

Prior to resecting the carpus, a guide (not shown) is used to measurethe carpal resection. The bones of the carpus are resected across theproximal carpal row. Preferably, the carpal resection is performed alonga plane generally perpendicular to the long axis of the radius. Thelunate, triquetrum and proximal scaphoid and the head of the capitatemay be resected. However, preferably only the lunate and the proximalpole of the scaphoid are removed. After the carpal bones are resected, asecond guide (not shown) is temporarily attached to the carpal bones inorder to allow for accurate drilling into the center of the capitate toprovide a starter hole for a cavity for carpal stem 84. In addition, twostarter holes are created to accommodate the placement of carpal screws38 into the carpus. The radius component 32, carpal ball 34 and carpalplate 36 are then articulated. To articulate carpal ball 34 to carpalplate 36, carpal ball 34 is pressed against ball 86 so that socket 74overlies ball 86. Force is then applied so that UHMWPE of carpal balldeforms around ball 86 and resiliently snaps back to grip ball 86.

The foregoing provides an overview of the surgical implantationprocedure. A detailed description of the surgical procedure can be foundbelow.

Once implanted, radius component 32 articulates with carpal ball 34 toprovide a simulation of natural motion of the wrist. The toroidal shapeof articular cup 48 and ellipsoidal shape of convex articular surface 68interact to provide a free and natural primary articulation of wristimplant 30. Primary articulation 40 preferably allows a minimum radialdeviation of about twenty degrees and an ulnar deviation of twentydegrees for a total of about forty degrees. Primary articulation 40further allows extension of the wrist at least forty degrees and flexionof the wrist at least forty degrees for a total of about eighty degreesminimum. Note that the wrist implant 30 of the invention as depicted anddisclosed herein permits much greater free movement of the wrist thanthese minimums. These minimum deviations have been found to provide agood range of motion for normal daily activities.

Thus, the motion of the implanted wrist implant 30 is generally hingeddorso-palmarly and gliding radio-ulnarly emulating normal jointkinematics. Flexion and extension occur about a first axis of rotationand radio-ulnar motion occurs about a second axis of rotation. In oneembodiment of the invention, radius component 32 articulates with carpalball 34 with a degree of incorporated laxity in the rotational degree offreedom. This incorporated laxity reduces torsional stresses that mightotherwise be transferred to the bone-implant interfaces and tend tocause loosening of radius component 32 or carpal plate 36.

In one embodiment of the invention, radius component 32 articulates withcarpal ball 34 in radial-ulnar deviation, flexion-extension and also ina rotational degree of freedom. As discussed above, convex articularsurface 68 may be generally ellipsoidally shaped to interface with thetoroidal shape of articular surface 64 of articular cup 48 to provide aclose sliding fit therewith near the center of convex articular surface68 but a looser sliding fit near articular ends 70. This interfaceallows carpal ball 34 to “wobble” relative to articular cup 48 thusproviding limited rotational movement about an axis generally along thelong axis of the radius. The use of ellipsoidal-toroidal geometrydemonstrates constant line to line contact in the anterior-posterior andlateral planes.

Carpal ball 34 articulates with carpal plate 36 at secondaryarticulation 42. Socket 74 is an interference fit with ball 86. Thissecondary articulation 42 provides for motion about a rotational axisgenerally parallel to the long axis of the third metacarpal. Secondaryarticulation 42 provides for more natural wrist motion and lessens therisk of loosening of carpal plate 36 from the carpus and radiuscomponent 32 from the radius by minimizing the application of torsionalforces to the bone-implant interfaces. The spheroidal head 102 of carpalscrews 38 limits secondary articulation 42 by the interaction of ovalexcavation 76 with head 102 of carpal screws 38. Preferably, thislimitation of movement is to about plus or minus five degrees or a totalof about ten degrees.

Tertiary articulation 44 arises between the head 102 of carpal screws 38and spheroidal countersink 94 in carpal plate 36 (FIG. 18). Tertiaryarticulation 44 allows the surgeon to angle carpal screws 38 as desiredfor best fixation in bony structures while at the same time assuringthat carpal screw 38 will have a tight interface with spheroidalcountersink 94. Tertiary articulation preferably allows about ten tofifteen degrees of variation in the angle of carpal screws in anydirection from a perpendicular to buttress surface 88.

Surgical Technique

A pre-operative assessment using an x-ray template should be made toapproximate the size of the wrist implant 30. A carpal cutting guide maybe provided with a small, medium, and large flange that can be fittedover the guide handle and tightened in place. Based upon an x-rayassessment, the appropriate size carpal resection guide should beassembled. The guide flange of the carpal resection guide is placedagainst the distal surface of the radius in the lunate fossa and theamount of carpal bone resection required is determined.

Dorsal Longitudinal Incision

A dorsal incision is made in line with the third metacarpal centereddirectly over Lister's tubercle.

Extensor Retinaculum Exposure

The extensor retinaculum is exposed and reflected from radial to ulnarfrom the first extensor compartment to the fifth or sixth extensorcompartment.

A midline incision over the fourth extensor retinaculum is acceptable in“dry” rheumatoids and post-traumatic or osteoarthritic wrist. The distalpart of the extensor retinaculum can be used to reinforce the dorsalwrist capsule in synovitic rheumatoid wrist.

Synovectomy of Extensor Tendons

After exposure, a synovectomy of the extensor tendons is performed asnecessary.

Exploration of Carpals

A rectangular shaped wrist carpal flap is reflected from proximal todistal to expose the proximal and distal carpal rows. Synovectomy ofwrist is performed as required.

Carpal Resection

A carpal resection guide (not shown) is set across the wrist joint forresection of the proximal carpal row. The length of the resection isdetermined by placing the flange against the distal radius/lunate fossaewithout carpal dislocation. The lunate, triquetrum, proximal scaphoid,and head of the capitate are resected.

Proximal Guide Placement

A proximal radial guide (not shown) is inserted to determine theresection of heterotopic bone and osteophytes from the distal radius.The convex side of the guide is placed against the concave surface ofthe distal radius. The wrist is flexed to allow best alignment of theproximal guide.

Drilling of the Radius

A radial template (not shown) in small, medium, and large sizes withleft and right hand configurations is utilized. The proper template tobe used is based upon the preoperative assessment. The distal face ofthe template represents the distal extent and peripheral coverage of theradial implant component. A drill hole in the template establishes astarting location for a broach. A 3.5 mm drill is inserted into the holeand a hole is drilled to a depth of 20-30 mm. The drill should bealigned along the long axis of the radius in both anterior/posterior andlateral planes. Biplanar fluoroscopic X-ray imaging confirmation ofguide placement is important to insure proper alignment prior tobroaching.

Preparation of the Radius

Based upon the pre-operative assessment of the implant size, the distalradius is broached with increasing sized broaches to allow full seatingof the radius component 32. Care should be taken to ascertain that thehandle of the broach is aligned with the long axis of the radius. Thebroach may need to be withdrawn occasionally to clean the teeth andclear the intra-medullary cavity of debris.

The goal is to remove no more subcortical bone than necessary to allowprosthesis insertion (resection of the distal radius, as is performedwith other wrist prostheses, is not required or recommended).

Radial Trial Placement

A trial radius component 32 is inserted into the prepared canal andimpacted. The fit of the radius component 32 against the scaphoid andlunate fossae is evaluated. If the fit is satisfactory, the trial radiuscomponent 32 is removed by engaging the extraction holes with a clamp.In some situations, it may be necessary to use a small burr to contourthe cartilage of the radius to achieve a desired fit. The radialtemplate (not shown) can be used as a guide to approximate the amount ofburring needed.

Carpal Templating

A carpal template (not shown) for locating the carpal plate 36 fixationholes for receiving carpal screws 38 is placed against the distal carpalresection site. A central hole in the template is aligned with thecenter of the capitate. Biplanar imaging can be used to confirm properalignment. Using k-wire(s), the distal pole of the scaphoid may betemporarily fixed to the capitate to facilitate scaphoid screwpreparation and insertion.

Carpal Drilling

A 3.5 mm drill is inserted into the central hole and a hole is drilledthrough the capitate but not into the second or third metacarpal.Imaging can be used to confirm proper alignment. The hole is drilled todepths appropriate for the size of the implant utilized. The drill bitis preferably disconnected from the drill driver and left in place tofacilitate drilling of the radially and ulnarly positioned screw sites.A 0.62 k-wire is inserted into the ulnarly located hole of the carpaltemplate. The k-wire can be angulated within the carpal template toachieve proper placement within the distal carpal row. The k-wire isdrilled to the depth of the screw length determined pre-operatively andthe drill is removed from the k-wire. Imaging can be performed toevaluate placement. The same procedure is repeated for the radiallypositioned hole of the carpal template. If the guide is alignedcorrectly, the k-wires and drill bit should form a “W” shape within thedistal carpal row.

Carpal Trial Placement

The stem of the trial carpal plate 36 is inserted into the capitateverifying that the dorsal aspect of the trial component is positionedcorrectly. Note: the stem is offset dorsally from the screw holes toaccommodate the natural arch of the carpus. The trial carpal plate isseated against the resection using a carpal impactor (not shown).Placement is confirmed with imaging.

Radial Trial Placement

The trial radial component 32 is placed into the radius and tapped intoplace using an impactor (not shown).

Carpal Ball Trial Placement

A carpal ball 34 trial component is placed over the trial carpal plate36 and the joint is reduced. Note: preferably the carpal balls 34 areavailable in two thicknesses, neutral and plus.

Range of Motion Assessment

The joint is articulated and joint stability and motion are assessed.There should be slight distraction across the implant interface of nomore than 2-3 mm. The cut generated by the carpal resection guide isdesigned for the neutral thickness of the carpal ball 34. However, ifthere appears to be too much joint laxity, the plus thickness carpalball 34 can be used. It is very important that the proper carpal ball 34size is selected because it is difficult to disarticulate the carpalball 34 from the carpal plate 36 once assembled. Full range of motionwithout impingement or instability should be present. Once range ofmotion is satisfactory, all trial components should be removed.

Component Placement

The definitive radius component 32 and carpal plate 36 and carpal ball34 are now inserted. The radius component 32 is designed for press fit,however bone allograft can be added if there is osteopenia orosteoporosis and to adjust seating of the radius component 32 to make upfor any joint laxity. The radius component 32 is tapped firmly intoplace. Optionally, bone cement can be used to secure the radiuscomponent 32. The carpal plate 36 is inserted next. It is aligned withthe centering hole in the capitate and pushed or tapped into place. Theself-tapping radial and ulnar cancellous carpal screws 38 are insertedthrough the carpal plate 36 and into the holes created by the k-wires.The screws are now tightened into place.

The carpal ball 34 is snapped into place using the carpal ball impactor(not shown). The radius component 32 and carpal plate 36 are reduced andarticulated. Any temporary scaphoid to capitate k-wires are removed. Ifindicated, bone graft from excised carpal bones can be used to fusetogether the distal carpal bones.

If the carpal ball 34 needs to be removed, care must be taken to notdamage the polished secondary articular surface 90 of the carpal plate36. Removal may be necessary to adjust joint tension or laxity or tomodify screw length. This can be accomplished by drilling a small holein the radial and ulnar flattened articular ends 70 of the polyethyleneand engaging the holes created with a bone reduction forceps and pryingin a radial or ulnar direction to disengage the snap fit assembly.

Repair of Dorsal Capsule & Closure

The dorsal capsule is repaired back to the distal radius. If necessarydrill holes are made in the dorsal, and distal radius. A tight capsuleclosure is performed with the wrist in extension (20°). A distal thirdof extensor retinaculum is added to reinforce capsular closure. Lastly,the extensor retinaculum is closed.

The present invention may be embodied in other specific forms withoutdeparting from the central attributes thereof, therefore, theillustrated embodiments should be considered in all respects asillustrative and not restrictive, reference being made to the appendedclaims rather than the foregoing description to indicate the scope ofthe invention.

1. An implantable wrist prosthesis for implantation in a wrist jointcomprising: (a) a radius component having an articular receiver and anintramedullary stem integral with and extending from the articularreceiver, the articular receiver having a concave inner wall and furtherhaving a convex outer wall, the convexity of the outer wall generallymatching a concavity of an unresected distal portion of a patient'sradius bone, the concavity of the inner wall of the articular receivergenerally matching a contour of a toroid; (b) a carpal plate; and (c) acarpal ball situated between the radius component and the carpal plate,the carpal ball comprising a convex surface having a major radius and aminor radius that engages and articulates with the concave inner wall ofthe articular receiver of the radius component.
 2. The implantable wristprosthesis of claim 1, wherein the carpal plate further comprises abuttress surface and a secondary articular surface.
 3. The implantablewrist prosthesis of claim 2, wherein the buttress surface issubstantially flat.
 4. The implantable wrist prosthesis of claim 2,wherein the secondary articular surface comprises a generally spheroidalprotrusion.
 5. The implantable wrist prosthesis of claim 1, wherein thecarpal ball further comprises a back surface comprising a generallyspheroidal socket.
 6. The implantable wrist prosthesis of claim 5,wherein the back surface of the carpal ball is curved.
 7. Theimplantable wrist prosthesis of claim 5, wherein the generallyspheroidal socket comprises a rim having a first diameter and aspheroidal portion having a second diameter.
 8. The implantable wristprosthesis of claim 7, wherein the first diameter is less than thesecond diameter.
 9. The implantable wrist prosthesis of claim 1, whereinthe carpal ball engages the carpal plate via an interference fit betweena generally spheroidal protrusion on the carpal plate and a generallyspheroidal socket on the carpal ball.
 10. The implantable wristprosthesis of claim 1, wherein the carpal plate comprises a stem. 11.The implantable wrist prosthesis of claim 1, wherein the carpal platecomprises at least one carpal screw.
 12. The implantable wristprosthesis of claim 1, wherein the carpal ball has at least two degreesof freedom relative to the articular receiver, the carpal ball engagingthe carpal plate and having limited rotational freedom relative to thecarpal plate, the carpal ball engaging the carpal plate via aninterference fit between a generally spheroidal protrusion on the carpalplate and a generally spheroidal socket on the carpal ball.
 13. Theimplantable wrist prosthesis of claim 1, wherein the concave inner wallof the articular receiver further comprises a major radius and a minorradius.
 14. The implantable wrist prosthesis of claim 13, wherein themajor radius of the concave inner wall and the major radius of theconvex surface are generally matching, and wherein the minor radius ofthe concave inner wall and the minor radius of the convex surface aregenerally matching.
 15. The implantable wrist prosthesis of claim 1,wherein a juncture between the intramedullary stem and the articularreceiver is configured to approximate anatomical volar tilt and ulnarinclination angles of the distal portion of the radius bone.
 16. Theimplantable wrist prosthesis of claim 1, wherein the carpal ballarticulates with the carpal plate about a rotational axis generallyparallel to the long axis of the third metacarpal.
 17. An implantablewrist prosthesis for implantation in a wrist joint comprising: (a) aradius component having an articular receiver and an intramedullary stemintegral with and extending from the articular receiver, the articularreceiver having a concave inner wall and further having a convex outerwall, the convexity of the outer wall generally matching a concavity ofan unresected distal portion of a patient's radius bone, the concavityof the inner wall of the articular receiver generally matching a contourof a toroid; (b) a carpal plate comprising: 1) a substantially flatbuttress surface; 2) a secondary articular surface comprising agenerally spheroidal protrusion; 3) a stem integral with the plate; and4) at least one carpal screw; and (c) a carpal ball situated between theradius component and the carpal plate, the carpal ball comprising aconvex surface having a major radius and a minor radius that articulateswith the concave inner wall of the articular receiver of the radiuscomponent and a back surface comprising a generally spheroidal socket,wherein the carpal ball engages the carpal plate via an interference fitbetween the generally spheroidal protrusion and the generally spheroidalsocket, further wherein the carpal ball has at least two degrees offreedom relative to the articular receiver, and further wherein thecarpal ball has limited rotational freedom relative to the carpal plate.18. The implantable wrist prosthesis of claim 17, wherein a juncturebetween the intramedullary stem and the articular receiver is configuredto approximate anatomical volar tilt and ulnar inclination angles of thedistal portion of the radius bone.
 19. The implantable wrist prosthesisof claim 17, wherein the carpal ball articulates with the carpal plateabout a rotational axis generally parallel to the long axis of the thirdmetacarpal.
 20. An implantable wrist prosthesis for implantation in awrist joint comprising: (a) a radius component having an articularreceiver and an intramedullary stem integral with and extending from thearticular receiver, the articular receiver having a concave inner wall,the concavity of the inner wall generally matching a contour of atoroid; (b) a carpal plate; and (c) a carpal ball situated between theradius component and the carpal plate, the carpal ball comprising aconvex surface having a major radius and a minor radius that engages andarticulates with the concave inner wall of the articular receiver of theradius component, the carpal ball articulating with the carpal plateabout a rotational axis generally parallel to the long axis of thepatient's third metacarpal.